Examples of prior art differential gears are disclosed in Japanese Published Unexamined (Kokai) Utility Model Laid Open Nos. 68-78746 and 62-172842, as shown in FIGS. 1A and 1B, respectively.
In FIG. 1A, the prior art differential gear 201 comprises a bevel gear type differential gear mechanism 207, a viscous coupling 211 for limiting differential motion, and a multiple disk clutch 215. The viscous coupling 211 is differential limiting means for generating a differential limiting force which increases with increasing number of differential rotations. Further, the multiple disk clutch 215 is engaged on the basis of a reaction force generated due to mesh between a side gear 219 and a pinion gear 223. Since this mesh reaction force between two gears 219 and 228 increases with increasing transmission torque of the differential gear mechanism 207, the multiple disk clutch 215 is a kind of differential limiting means of torque sensitive type.
In the above-mentioned differential gear 201, on the other hand, the left side gear 219 is connected to a left wheel shaft 288 via a hub 231, and a right side gear 235 is directly connected to a right side wheel shaft 237, respectively. Further, the multiple disk clutch 215 is interposed between the hub 231 and the differential casing (or housing) 239, so that the multiple disk clutch 215 is of S (shaft)-H (housing) type.
In the same way, in FIG. 1B, the prior art differential gear 203 comprises a bevel gear type differential gear mechanism 209, a viscous coupling 213 for limiting differential motion, and a multiple disk clutch 217. The viscous coupling 213 is differential limiting means for generating a differential limiting force which increases with increasing number of differential rotations. Further, the multiple disk clutch 217 is engaged on the basis of a reaction force generated due to mesh between a side gear 221 and a pinion gear 225. Since this mesh reaction force between two gears 221 and 225 increases with increasing transmission torque of the differential gear mechanism 209, the multiple disk clutch 217 is a kind of differential limiting means of torque sensitive type.
In the above-mentioned differential gear 203, on the other hand, the multiple disk clutch 217 is interposed between the right side gear 221 and a differential casing (housing) 241, and the viscous coupling 213 is also interposed between the side gear 243 and the differential casing (housing) 241, respectively. Therefore, the multiple disk clutch 217 is also of S (shaft)-H (housing) type.
Here, in the differential gear 201, the transfer ratio R.sub.tL of the left side wheel shaft 233 and the transfer ratio R.sub.tR of the right side wheel shaft 237 can be expressed as follows: ##EQU1## where F.sub.L denoted a frictional torque generated by the multiple disk clutch 215 according to a magnitude of an input torque and then transmitted to the left side wheel shaft 233; F.sub.R denoted a frictional torque generated by the multiple disk clutch 215 according to a magnitude of an input torque and then transmitted to the right side wheel shaft 237, respectively.
Here, if f.sub.L =0, ##EQU2##
Here, since f.sub.R .noteq.0, R.sub.tL .noteq.R.sub.tR, thus causing an unbalanced or non-uniform transfer ratio.
In the case of the differential gear 201 as shown in FIG. 1A, engine power is transmitted from the differential casing 239 to the left and right side wheel shafts 233 and 237 as shown by thick solid lines 249, and further a driving power as shown by dashed lines in FIG. 1A is also transmitted to the right side wheel shaft 237.
In summary, when the differential limiting means is interposed in way of the above-mentioned S-H arrangement type, there exists a problem in that the differential limiting force is not uniform On the right and left side wheel shafts.
The above-mentioned non-uniform power transmission will be described in more detail with reference to FIG. 1C-1, which is a graphical representation showing the differential limiting characteristics of the differential gear 201. In FIG. 1C-1, the ordinate represents a torque transmitted to the right wheel shaft under condition that the left wheel shaft is skidding or idling, and the abscissa represents a torque transmitted to the left wheel shaft under condition that the right wheel shaft is skidding or idling. In the graph shown in FIG. 1C-1, the straight line 253(R) indicates the right wheel shaft torque obtained through the multiple disk clutch 215, and the straight line 255(L) indicates the left wheel shaft torque obtained through the multiple disk clutch 215, respectively. These straight lines 253(R) and 255(L) indicate that the left and right wheel shaft torques are not symmetrical with respect to the straight line 257 with a gradient of 45 degrees; that is, the right wheel shaft torque 253(R) is larger than the left wheel shaft torque 255(L). In FIG. 1C-1, the ratio of the gradient .theta..sub.R of the straight line 253(R) from the abscissa to the gradient .theta..sub.L Of the straight line 255(L) from the ordinate is referred to as transfer ratio (Rt).
Further, in FIG. 1C-1, when a uniform differential limiting force is applied to both the above-mentioned wheel shaft torques indicated by the lines 253(R) and 255(L) through the viscous coupling 211, the straight line 253(R) indicative of the right wheel shaft torque is shifted to the straight line 259(R), and the straight line 255(L) indicative of the left wheel shaft torque is shifted to the straight line 261(L), respectively into asymmetrical relationship with respect to each other.
FIG. 1C-2 is a graphical representation showing the differential limiting characteristics of the differential gear 203 shown in FIG. 1B. In FIG. 1C-2, the ordinate represents a torque transmitted to the right wheel shaft under condition that the left wheel shaft is skidding or idling, and the abscissa represents a torque transmitted to the left wheel shaft under condition that the right wheel shaft is skidding or idling. In the graph shown in FIG. 1C-2, the straight line 263(L) indicates the left wheel shaft torque obtained through the multiple disk clutch 217, and the straight line 265(R) indicates the right wheel shaft torque obtained through the multiple disk clutch 217, respectively, which is opposite to those shown in FIG. 1C-1. This is because the multiple disk clutch 217 is interposed on the side of the right side gear 22 in way of the S-H arrangement type. These straight lines 263(L) and 265(R) indicate that the left and right wheel shaft torques are not symmetrical with respect the straight line 257 with a gradient of 45 degrees; that is, the left wheel shaft torque is larger than the right wheel shaft torque. In FIG. 1C-2, the ratio of the gradient .theta..sub.L of the straight line 263(L) from the abscissa to the gradient .theta..sub.R of the straight line 265(R) from the ordinate is referred to as transfer ratio (R.sub.t).
Further, in FIG. 1C-2, a uniform differential limiting force is applied to both the above-mentioned wheel shaft torques indicated by the lines 263(L) and 265(R) through the viscous coupling 213, the straight line 263(L) indicative of the left wheel shaft torque is shifted to the straight line 267(L), and the straight line 265(R) indicative of the right wheel shaft torque is shifted to the straight line 269(R), respectively into asymmetrical relationship with respect to each other.
In summary, in the conventional differential gear provided with both torque sensitive and speed sensitive characteristics, it has been impossible to obtain the differential limiting characteristics uniform on both right and left sides of the wheel shafts.